1. Field of the Invention.
The present invention relates to charge pumps used as loop filters in phase locked loops.
2. Description of the Prior Art.
Phase locked loops are used widely in data communications, disk and tape drives, frequency synthesizers, and many other areas of modern electronics. Phase locked loops synchronize a variable local oscillator with the phase of a transmitted signal, thus allowing, for example, recovery of the clock of the transmitted signal.
A phase locked loop is typically constructed from a phase detector and a voltage controlled oscillator. The phase detector detects the phase difference between a clock signal and the incoming transmitted data signal. Provision is made for adjusting a control signal in proportion to the phase difference. The voltage control oscillator generates the clock signal, the frequency of which varies as a function of the control signal. A second order phase locked loop further includes a filter between the phase detector and the voltage controlled oscillator. The filter can be used to prevent the phase locked loop from capturing and locking onto harmonics of the transmitted signal and for increasing the stability of the recovered clock. The filter can also be used to define operational bandwidth for the device.
Charge pump type filters for phase locked loops are popular for filtering in "baseband" data communication, i.e. direct data pulse transmission. They are used for recovery of the clock of the transmitted data. With charge pumps in a phase locked loop, the phase detector is typically adapted to generate "UP" pulses on a first output terminal when the phase of the incoming data leads the phase of the clock, and "DOWN" pulses on a second output terminal when the phase of the clock leads the phase of the incoming clock. A cumulative duration of "UP" pulses exceeding the cumulative pulse duration of the "DOWN" pulses indicates that the frequency of the output signal of the voltage controlled oscillator is to be increased. "DOWN" pulse cumulative duration in excess of the "UP" pulses indicates that the frequency of the output signal of the voltage controlled oscillator is to be decreased.
In prior art devices, the UP and DOWN pulses are used to generate relatively positive and negative pulses of current. The difference of the integrals of the positive and negative pulses of current is proportional to the detected phase difference. A capacitor is provided to integrate the opposite polarity current pulses and thereby indirectly provide a control signal for the voltage controlled oscillator.
An ideal charge pump exhibits:
(1) equal amplitude UP and DOWN current;
(2) symmetric transient characteristics between the UP and DOWN currents; and
(3) a steady state output, i.e. equal duration UP and DOWN currents, when the phase detector indicates matched phase between the voltage controlled oscillator and the transmitted signal.
Such characteristics have not been readily obtained in prior art circuits, particularly when such circuits have been carried over into solid state implementation. U.S. Pat. No. 4,636,748 issued Jan. 13, 1988 to Latham, II, noted that prior art charge pumps, which used complementary current sources to provide the opposite polarity UP and DOWN currents, required matched operating characteristics in the NPN and PNP transistors. Realization of matching characteristics between NPN and PNP devices embodied in the monolithic integrated circuit is extremely difficult. Devices so constructed have given unsatisfactory performance. In the present state of the art, integrated PNP transistors are inferior to NPN devices, particularly in terms of frequency response.
Latham II, proposed an all NPN charge pump. The Latham pump uses a single current source and two identical current switching paths arranged in a bridge circuit. Each arm of the bridge circuit includes an NPN transistor switch. A capacitor is connected between a pair of opposed nodes in the bridge. A voltage source and a current source, respectively, are connected to the remaining two nodes. The UP and DOWN pulses are applied to transistor switches in opposed arms of the bridge, respectively. An UP pulse is applied to one of the transistor switches connecting the voltage source to one terminal of the capacitor and to the transistor switch connecting the opposite terminal of the capacitor to the current source. A DOWN pulse is applied to the second transistor connecting the voltage source to a terminal of the capacitor and to the transistor connecting the opposite terminal of the capacitor to the current source. In other words, UP and DOWN pulses result in switching in the bridge circuit to form symmetric current paths across the bridge and, in opposite directions, through the capacitor. An UP pulse charges the capacitor, a DOWN pulse discharges the capacitor. Simultaneous UP and DOWN pulses, or the absence of either, leave the charge on the capacitor unaffected. The phase locked loop filter is connected across the opposite terminals of the capacitor, being adapted to provide a control signal to the voltage controlled oscillator from the differential signal.